Chemical Order- induced magnetic exchange bias

D. Lott (GKSS), F. Klose (ANSTO), H. Ambaye (ORNL), G.J. Mankey & P. Mani (Univ. of Alabama), M. Wolff (ILL), A. Schreyer (GKSS), H.M. Christen & B.C. Sales (ORNL)

In recent years, significant research efforts went into exploring the exchange bias effect in thin film structures because of important spin electronics applications. While the basic mechanisms leading to exchange bias are qualitatively well understood, a quantitative understanding of the effect is, in view of complex chemical structures and electronic interactions at actual FM/AFM thin film interfaces, often challenging.

So far, only exchange bias combinations of two different materials with corresponding non-perfect interfaces have been investigated simply because, in order to show AFM and FM magnetic phases at the same temperature, the two materials usually need to be chemically different.

In this regard, FePt3 is a remarkable exception because, depending on the degree of chemical order, it can, at the same temperature and composition, develop either FM or AFM magnetic order.

We have studied the exchange bias effect of an epitaxial multilayer stack consisting of FePt3 layers with partial chemical order and FM CoPt3 layers. Polarized neutron reflectometry (PNR) measurements show that at low temperature the FePt3 layers are in a magnetic state with both AFM and FM domains present.

The AFM part appears to be paramagnetic at high temperature and, based on AC susceptibility measurements, must have a N?el transition at about 160 K. Our experimental results indicate that the exchange bias effect is created within the FePt3 layers, at the interface between the chemically ordered and disordered FePt3 domains.

This is the first experimental evidence that a chemical ordering process within a single crystalline material of homogeneous composition is causing exchange bias.

*Phys. Rev. B 77 (2008) 132404.

FIG. 1.    Temperature-dependent exchange bias of the FePt3/CoPt3 superlattice. Insert: AC susceptibility demonstrating the AFM ordering in the FePt3 layers.

FIG. 2. Temperature-dependent polarized neutron reflectometry measurements allow to determine of the layer-resolved magnetization profile of the FePt3/CoPt3 superlattice.

FIG. 3. (Layer-resolved) volume magnetizations determined by PNR  and their comparison with SQUID data.